The present invention relates to radio communications systems, and in particular to the handover procedure in cellular mobile radio systems.
In a cellular mobile telecommunications network, a mobile unit communicates with a base station covering the area, or cell, in which the mobile unit is found. Around this cell, there are other cells, covered by other base stations. In some situations, a mobile unit needs to transmit signals to a base station, or a base station to a mobile station, in the form of repeated bursts each burst comprising the same information.. Usually, this is done to increase the probability of the base station, or the mobile unit, successfully receiving and interpreting the signal.
In a Global System for Mobile Communication (GSM), the most common situations in which such repeated bursts are transmitted are handover, replies to polling requests and certain acknowledgement signalling when packet switching is used. These situations will be described in the following.
When a mobile unit having an established connection to a base station moves from the cell of this base station to another cell, the base station in the new cell should take over the connection from the first base station. This procedure is called handover. To make the handover procedure faster, the mobile unit constantly monitors the current and other cells and reports measurement data for these cells to the network. The base station controller decides if the mobile unit should change its connection from the current base station to another base station. If this is found to be the case, the mobile unit sends a number of access messages to the new base station. When the new base station acknowledges that an access message has been received, the new connection can be established and the previous one disconnected. The Fast Associated Control Channel (FACCH) is a logical control channel used in GSM for, among other things, signalling in connection with handover.
Handover is a critical phase of a connection as it is often performed when a mobile unit is far away from base stations, that is, when the signal properties are poor. Furthermore, according to some standards, for example, GSM, some signalling is required on the speech/data channel, involving a break in the speech or data. The handover procedure therefore should be made as fast and reliable as possible and have a high performance with respect to noise and interference.
When packet switching is used, for example according to the Global Packet Radio Services (GPRS) standard in GSM, a polling request is transmitted from a base station to a mobile unit to check if the mobile unit is present. The mobile unit responds by transmitting four identical bursts, that are used by the base station to determine the signal strength and other parameters needed when communicating with the mobile unit.
Also, when packet switching is used, an acknowledgement signal is transmitted from the base station when a message packet has been successfully received. This acknowledgement signal is repeated until it has been successfully received by the mobile unit. When the mobile unit receives such an acknowledgement signal, it therefore responds by transmitting a signal acknowledging that the acknowledgement signal has been received. This xe2x80x9cacknowledgement of acknowledgementxe2x80x9d signal comprises four bursts, each burst comprising the same information.
In all of these situations of repeated bursts being transmitted, it is desirable that the information is successfully received and interpreted as quickly as possible, as the transmission of these repeated bursts inhibits other communication.
These situations are particularly critical when antennas are used that do not receive information from all directions at the same time, especially if the position of the mobile unit is unknown. The signal strength then varies with time in dependence of the directions in which the antenna is receiving at a given time.
Repeated bursts of information are also transmitted from the base station to the mobile unit, for example when ordering handover. Also, system information on the Broadcast Control Channel (BCCH) and synchronization information on the Frequency Correction Channel (FCCH) and the Synchronization Channel (SCH) are transmitted in the form of repeated bursts.
A solution for making the reception of a signal more reliable is disclosed, for example, in U.S. Pat. No. 5,390,166. Here each information burst is transmitted simultaneously from two different transmitters using two different carrier frequencies, and the two bursts are combined at the receiving end. This solution requires two transmitters transmitting the same information and thus is not suitable for a cellular communication system having a limited frequency spectrum.
In Total Access Communications Systems (TACS) and Advanced Mobile Phone Service (AMPS) systems, the reliability of the reception of information is achieved by transmitting a word, or block of data, which has first been encoded, for example, five or eleven times. U.S. Pat. No. 5,995,559, describes how the five uplink repetitions of a word are received and decoded. If one of the repetitions is successfully decoded, the remaining repetitions are ignored. If none of the repetitions can be successfully decoded, a bit-wise majority vote of the five repetitions is taken and the voting result is decoded. The bit values determined in this way are deemed to constitute the received word, which is passed to a decoder.
U.S. Pat. No. 5,568,513 describes how the repetition of words can be exploited in the AMPS and TACS systems. This is for instance performed by having a set of windows selecting a resulting word from the parts of each repeated word having sufficient quality. In another aspect of U.S. Pat. No. 5,568,513 the resulting decoded word comes from a so-called majority voting of the repeated decoded words. U.S. Pat. No. 5,568,513 is strictly based on combinations or selections of decoded bits, that is, on combination on a digital level.
It is an object of the present invention to make the reception of information transmitted as repeated bursts in a radio communications system faster and more reliable.
It is another object of the present invention to enable a fast and reliable handover procedure in cellular communications networks.
These objects are achieved according to the present invention by a receiving device for use in a radio communications system in which certain information is received in the form of repeated bursts comprising the same information, said receiving device comprising means for receiving and processing incoming signals from a transmitting device in the same radio communications system, said receiving device comprising means for receiving at least two and temporarily storing at least one received repeated burst, before they are decoded, and equalizing and combining means for equalizing and combining said received repeated bursts.
The receiving device can also comprise duplicated receiving means for receiving signals from a transmitter in the radio communications system and combining means for combining the signals received by the duplicated receiving means.
The receiving device can be a part of a radio base station or a terminal in a radio communications system.
A method of receiving information being transmitted from a transmitting unit to a receiving unit in a radio communications system in the form of repeated bursts comprising the same information is also disclosed, said method comprising the following steps:
receiving and temporarily storing said repeated bursts;
equalizing and combining the information of at least two repeated bursts before decoding;
interpreting the equalized and combined information in the same way as the information of a single burst would be interpreted.
In a preferred embodiment, bitwise combination is used, and more specifically a Maximum Ratio Combining algorithm.
Weights, can be assigned, bit-wise or burst-wise, to the received signal in dependence of the signal quality.
The invention offers the following advantages:
The reliability of the information received in the form of repeated bursts can be increased without duplication of antennas or other equipment.
The time needed to receive such information successfully can be reduced without duplication of antennas or other equipment.
The disadvantages of antennas that do not receive from all directions at the same time are reduced.